Most imaging systems typically employ a single focus point, at which focusing is optimum. While such systems can result in a sharply focused image when the object to be imaged is at the focus point, such systems are typically sensitive to variations in the distance between the object to be imaged and the imaging system or more particularly its focusing lens. While it is well known to increase the depth of field of a well-focused lens system by decreasing the aperture of the system, that can severely decreases light collection efficiency, thereby possibly limiting the speed at which such a system can operate.
Other techniques for imaging with an extended focusing depth have been contemplated. For example, U.S. Pat. No. 5,371,361, which is assigned to the same assignee as is this invention, discloses an imaging system having a soft-focus lens, which sacrifices the quality of mid-field focus to achieve near invariance of focus throughout a range of distances, in addition to equalization of the electronic image signal. As another example, U.S. Pat. No. 5,748,371 and related works by the inventors of that patent disclose a combination of particular optics (cubic phase mask) and digital signal processing to provide an in-focus response over a wide range of object distances. The cubic phase mask has an optical transfer function that is relatively insensitive to object distance over a predetermined range, and the digital signal processing is designed to undo the effects of the cubic phase mask on the optical transfer function (other than increased depth of field). The inventors of that patent claim that jointly designing complementary cubic phase mask and digital signal processing can result in imaging results not possible with optical elements only. However, a cubic phase mask is a complicated, asymmetric part that is expensive and cumbersome to fabricate. Moreover, the asymmetry of a cubic phase mask requires that the complementary digital signal processing be performed over two dimensions.